To determine Mg excess, don’t look at how many pounds you have in the soil. Look at the base saturation percentage. Heavy soil may have far more magnesium in pounds than a light soil, but a heavy clay needs a lower overall percentage, and sand needs a higher percentage – on a heavy clay, Mg should be 10-12%; on lighter sand, it could ideally be as high as 20%.
The biggest limitation to me is aeration. We have to have that air. Improving soil health means building the right environment, starting with the proper amount of air and water. If we don’t have the right amount of air, we can add all the microbes, sunlight, and fertility we want, but it won’t make as much difference. Magnesium is one of the keys to aeration.
One of the first things farmers check on a soil test is the pH, but pH is not the answer – pH is an indicator. If you have a low pH, you can know you need limestone, but the composition of that lime varies depending upon if your soil is sandy or clay-based. If sandy, you generally need more emphasis on magnesium and less on calcium. If clay, you need calcium and sometimes magnesium. Often, clay soils just have a natural excess of Mg, so we try to reduce the percentage of it in the soil to have better porosity. However, Mg is not the full answer. Base saturation is principally expressed as a percentage of calcium, magnesium, potassium, sodium, and hydrogen. Apart from magnesium, two other elements that can damage porosity are potassium and sodium, and having any of these three in excess always restricts needed pore space in soil.
In clays, we need larger amounts of Ca to overcome high Mg. Calcium causes clay to gather in larger clumps or bigger particles. As clay particles are flocculated or pulled together, it increases pore space and aeration. In sandy soils, we have the opposite problem: too much air, not enough water. To get sandy ground to produce correctly, we need more Mg, less Ca.
A lot of times, you hear about the calcium-to-magnesium ratio, but in my opinion, this gives the wrong impression. If you use ratios, those don’t really apply until you’ve established your cation exchange capacity. A very productive light sandy soil will have a 3:1 Ca-to-Mg ratio, but you use Mg as a basis. In sand, if you have enough Mg, then determine the Ca you need. In heavy soils, this ratio is often more like 6.5:1 or 7:1, but again if the base you begin with is set as too high or too low, then the amount of both will still be either deficient or excessive.